WEDNESDAY, March 21 (HealthDay News) -- Pregnancy is safe for women with estrogen-receptor-positive breast cancer, according to a new study.

These types of breast tumors are especially sensitive to levels of estrogen in the body, and the findings address concerns that pregnancy could increase circulating estrogen and thereby cause the cancer to return.

This study's results suggest that becoming pregnant at any time after being diagnosed with breast cancer does not increase the risk of recurrence, even if the pregnancy occurs within two years after cancer diagnosis, according to the researchers.

In addition, breast-cancer patients who become pregnant appear to survive longer than those who do not, according to Dr. Hatem Azim Jr., a medical oncologist at the Jules Bordet Institute in Brussels, Belgium, and colleagues.

Their study included 333 breast-cancer patients who later became pregnant and 874 breast-cancer patients who did not. Over an average of nearly five years of follow-up, breast cancer recurred in 30 percent of all the women.

Fifty-seven percent of the women in the study had estrogen-receptor-positive breast cancer. Becoming pregnant made no difference in the length of time that patients with either estrogen-receptor-positive or estrogen-receptor-negative breast cancer survived without their cancer recurring, according to the study, which is slated for presentation Wednesday at the European Breast Cancer Conference, in Vienna, Austria.

"We found that patients who became pregnant within two years of breast-cancer diagnosis appeared to have a better disease-free survival compared to those who did not become pregnant," Azim said in a conference news release.

"However, a clear trend over time was not demonstrated; hence this finding should be interpreted with caution as it could be confounded by potential selection bias, and hence pregnancy within two years of diagnosis should be regarded as safe, and not as protective," Azim noted.

A secondary goal of the study was to determine the impact of pregnancy on overall survival.

"We found that breast-cancer patients who became pregnant also had a lower risk of death compared to their matched controls, irrespective of [estrogen receptor] status," Azim said.

Because this study was presented at a medical meeting, the data and conclusions should be viewed as preliminary until published in a peer-reviewed journal.